Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is not stored as electric charge, but is instead stored by magnetic polarization of storage elements. The storage elements are formed from two ferromagnetic layers separated by a tunneling layer. One of the two layers has at least one pinned magnetic polarization (or fixed layer) set to a particular polarity. The magnetic polarity of the other magnetic layer (or free layer) is altered to represent either a “1” (e.g., anti-parallel to the fixed layer) or “0” (e.g., parallel to the fixed layer). One such device having a fixed layer, a tunneling layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ is dependent on the magnetic polarity of the free layer compared to the magnetic polarity of the fixed layer. A memory device such as MRAM is built from an array of individually addressable MTJs.
FIG. 1 is a circuit schematic illustrating a portion of a conventional magnetic random access memory (MRAM). An MRAM 100 is divided into a number of bitcells 110, 140, 160. During read out of the bitcell 160, the resistance of the bitcell 160 is compared to the reference parallel bitcell 110 and the reference anti-parallel bitcell 140. Resistance of the bitcells 110, 140, 160 are measured by applying a source voltage and determining an amount of current flowing through the bitcells 110, 140, 160. For example, in the bitcell 110, a voltage source 120 is applied to a magnetic tunnel junction (MTJ) 112 by read select transistors 122, 124, and a word line select transistor 126. The MTJ 112 includes a fixed layer 114, tunneling layer 116, and a free layer 118. When the free layer 118 and the fixed layer 114 have magnetizations aligned substantially parallel, the resistance of the MTJ 112, and thus the bitcell 110, is low. When the free layer 118 and the fixed layer 114 have magnetizations aligned substantially anti-parallel, the resistance of the MTJ 112, and thus the bitcell 110, is high.
Data may also be stored in the MTJ 112 by passing current through the MTJ 112 to cause spin transfer torque (STT). Thus, when current is passed through the MTJ 112 for a read operation, the MTJ 112 may be subject to a read disturb event in which the stored value of the MTJ 112 is changed.
FIG. 2A is a graph illustrating a read disturb event thr a magnetic tunnel junction in an anti-parallel state. When a current flows through a magnetic tunnel junction in an anti-parallel state from a free layer of the MTJ to a fixed layer of the MTJ, the MTJ is subject to a read disturb event. In a graph 200 a line 202 represents the resistance of an MTJ in an anti-parallel state as a function of current through the MTJ, where positive current denotes current flowing in a direction from the free layer to the fixed layer. A line 204 represents the resistance of a MTJ in a parallel state as a function of current through the MTJ.
When a read operation is performed on an MTJ in an anti-parallel state, current flowing through the MTJ may be at a point 206. At a point 208 current flowing through the anti-parallel state MTJ causes the MTJ to spontaneously switch to a parallel state. The region between the point 206 and the point 208 is the read disturb margin. Any variations in manufacturing, of the MTJ or associated circuitry may move the point 206 closer to the point 208. When the current exceeds point 208, the data stored in the MTJ is lost due to the read disturb event.
When current is applied in the positive direction through a parallel state MTJ, the MTJ is not subject to read disturb. A point 210 indicates the current passing through a parallel state MTJ during a read operation. Increasing the current through parallel state MTJ does not spontaneously switch the MTJ to an anti-parallel state. However, the parallel state MTJ is subject to read disturb when current flows through the MTJ in the opposite direction.
FIG. 2B is a graph illustrating a read disturb event for a magnetic tunnel junction in a parallel state. A point 220 indicates the current flowing through a parallel state MTJ during a read operation. A point 222 indicates a current level causing spontaneous switching of the MTJ from a parallel state to an anti-parallel state. The region between the point 220 and the point 222 is the read disturb margin for a parallel state MTJ. Any variations in manufacturing of the MTJ or associated circuitry may move the point 220 closer to the point 222. When the current exceeds point 222, the data stored in the MTJ is lost due to the read disturb event.
When current is applied in the negative direction through an anti-parallel state MTJ, the MTJ is not subject to read disturb. A point 224 indicates the current passing through an anti-parallel state MTJ during a read operation. Increasing the current through the anti-parallel state MTJ does not spontaneously switch the MTJ to a parallel state.
As the size of the MTJ and the bitcells in MRAM shrink to increase MRAM density, the read disturb margin further shrinks, and the MTJs are more frequently subject to read disturb events. Thus, there is a need for an MRAM device with reduced read disturb.